1. Field of the Invention
This invention relates to an optical disc having two layers of different rare earth-transition metal alloy films, and a manufacturing method for this optical disc.
2. Description of the Related Art
Among rewritable optical discs, there are a magneto-optical disc and a phase-change disc. In particular, with the magneto-optical disc, magnetic domains are written on magnetic thin films, by way of recording the information by exploiting the thermal energy, such as semiconductor laser, and the information thus recorded is read out using the photomagnetic effect.
As the recording materials used for this magneto-optical disc, films of rare earth-transition metal alloys, obtained on combining rare earth elements, such as Gd, Tb or Dy, and transition metals, such as Fe or Co, are predominantly used. As the magneto-optical disc employing the films of rare earth-transition metal alloys, such a magneto-optical disc is proposed in which the rare earth-transition metal alloy film is sandwiched between dielectric films of, for example, Si.sub.3 N.sub.4, and a reflective film of, for example, Al, is formed on one or the other dielectric film, to provide a four-layer structure.
Heretofore, the above-described magneto-optical disc is of a recording format in which a TbFeCoCr alloy film is used as the rare earth-transition metal alloy film and in which recording/reproduction is performed with a track pitch of approximately 1.6 .mu.m and a bit length of approximately 0.83 .mu.m.
Conventionally, with an increasing demand for higher recording density, such a recording format is proposed in which the track pitch is approximately 1.6 .mu.m and the pit length is approximately 0.83 .mu.m for recording/reproduction.
Also, there is recently raised a demand for higher recording density, such that a recording format which reduces the track pitch and the pit length further has been proposed. For example, in the above-described magneto-optical disc, the track pitch and the pit length are set to approximately 0.9 .mu.m and to approximately 0.45 .mu.m, respectively, in order to achieve higher recording density. However, if, in the above-described magneto-optical disc, the track pitch and the pit length are reduced further for achieving higher recording density, the playback output is lowered, such that recording/reproduction cannot be realized.
It may be envisaged to improve photomagnetic effects of the TbFeCoCr alloy film. With this magneto-optical disc, it suffices if the Kerr rotation angle of the rare earth-transition metal alloy film is increased for the laser light illuminated for playback. In order to realize this, it suffices to eliminate Cr from the TbFeCoCr alloy film, that is to use a TbFeCo alloy film. If the TbFeCo alloy film is used as a recording material, the Kerr rotation angle of the laser light illuminated on the recording layer is increased, as a result of which the playback output is improved.
However, this TbFeCo alloy film is not optimum in magnetic properties, as compared to a TbFeCoCr alloy film with an oxide film deposited thereon, such that a magnetic domain cannot be formed satisfactorily in a low external magnetic field. Thus, with a magneto-optical disc employing the TbFeCo alloy film as a recording material, recording cannot be made satisfactorily with an external magnetic field only large enough to make recording on the TbFeCoCr alloy film carrying an oxide film.
Also, the TbFeCo alloy film is formed by sputtering in an Ar gas atmosphere. If an oxygen gas is mixed, Tb of the TbFeCo alloy film is selectively oxidized, because Cr is not contained therein. If Tb is selectively oxidized in the TbFeCo alloy film, magnetic characteristics are deteriorated. That is, with the present TbFeCo alloy film, it is different to control the film thickness to form the oxide film, such that, if it is attempted to deposit an oxide film, an excessively oxidized state is produced. The result is that the magneto-optical disc has only poor recording/reproducing characteristics.
As a magneto-optical disc, developed to eliminate this inconvenience, there is known such a disc which has two sorts of the rare earth-transition metal alloy films. Specifically, the magneto-optical disc is of a layered structure of a GdFeCo alloy film and a TbFeCo alloy film. With this magneto-optical disc, the force of magnetic coupling by exchange reciprocal action operates between the GdFeCo alloy film and the TbFeCo alloy film, thus improving magnetic characteristics on the whole.
However, if a magneto-optical disc having two sorts of the rare earth-transition metal alloy film is actually produced, the exchange reciprocal action between the GdFeCo alloy film and the TbFeCo alloy film is not displayed satisfactorily such that the desired magnetic coupling force is not produced.